Showing that it's serious about quickly phasing out its venerable iDEN network and traditional push-to-talk service, Sprint Nextel now plans to charge all subscribers on that system an extra US$10 per month beginning Jan. 1.

Sprint has started informing subscribers of the upcoming charge, Sprint spokesman Mark Bonavia confirmed on Thursday. The cost of monthly plans under the Nextel and PowerSource brands will cost $10 more, Bonavia wrote in an email message. Plans for devices that run on Sprint's CDMA network won't be affected. "Customers that migrate prior to January will likely find a price plan comparable to what they have now," Bonavia wrote. The news originally had been reported by the Phone Scoop blog.

Sprint acquired the iDEN network when it merged with Nextel in 2005. Despite initial pledges to phase out the network in 2007 and bring Nextel's customers onto its CDMA system, Sprint kept iDEN going for several years. But the narrowband technology couldn't keep up with the age of mobile data, and two years ago Sprint said it would shut down iDEN in 2013. The network is set to go offline by June 30.

Nextel subscribers are leaving in large numbers, but at the end of the third quarter there were still 2.3 million postpaid and 800,000 prepaid users of the network. Sprint is trying to hold onto those departing customers as it continues to lag far behind AT&T and Verizon Wireless in total subscribers, with just 56 million. Sprint reported that 866,000 postpaid subscribers left Nextel during the third quarter but that 59 percent of them moved onto Sprint.

iDEN's strongest selling point is its push-to-talk feature, which workers in construction, manufacturing and other industries use to talk to each other instantly in walkie-talkie style. Sprint now offers Direct Connect, a service designed for the same uses on its 3G CDMA network. But its rivals want to use the Nextel shutdown to draw away former PTT users. On Thursday, AT&T introduced the Samsung Rugby III, a rugged flip phone that can use its Enhanced Push-To-Talk service. The Rugby III is set to go on sale Dec. 14 for $99.99 with a two-year contract.

The Nextel name will also go away in conjunction with Softbank's planned purchase of 70 percent of Sprint, which is expected to gain regulatory approval next year. After that deal goes through, Sprint has said it plans to drop "Nextel" from its company name.